Export read-only tcpip.Subnet.Mask
Export read-only tcpip.Subnet.Mask

PiperOrigin-RevId: 214023383
Change-Id: I5a7572f949840fb68a3ffb7342e6a3524bd00864

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor